This invention relates to sensing devices and methods of sensing. This invention also relates to capacitors in general. Particularly, the invention relates to apparatus and methods for sensing the volume of drug in a drug delivery device using capacitance.
Drug delivery devices are well known and used widely in the treatment of patients. One form of such delivery devices is an implantable infusion pump which typically includes an expansible reservoir for containing a refillable supply of drug. Flow control and refill features are also provided on the pump. Pumps of this type are disclosed in U.S. Pat. Nos. 4,692,147 to Duggan and 5,445,616 to Kratoska et al, the subject matter of which is incorporated herein by reference. State-of-the-art pumps typically contain a propellant that exists in liquid and vapor phases to maintain a constant vapor pressure on the drug reservoir, which is typically an expansible metal bellows. The vapor pressure of the propellant remains substantially constant as the volume of the bellows and therefore the volume of the space between the bellows and pump housing changes.
Frequently, throughout the life and use of a drug delivery device, it is advantageous to periodically determine the volume of drug contained in the drug reservoir. For example, when an implantable pump is refilled, it is beneficial for a physician or technician to know the precise volume of drug remaining in the reservoir and to detect when the pump has been completely refilled. Likewise, periodic volume readings during pump operation permit a physician or possibly even a patient to determine when refilling should be scheduled. Such volume data can also be used to determine or possibly predict pump or infusion system malfunctions or leaks.
Prior art techniques for determining reservoir volume tend to rely on labor and time intensive methods. For example, during refill, the residual supply of drug in the pump may be determined by evacuating, to the extent possible, the residual supply of drug in the pump. Given the amount of effort and time involved in known techniques, it would be advantageous to provide a device for quickly, easily and accurately sensing the volume of drug contained in the reservoir at any desired time.
The invention reveals a unique discovery: the concept of determining reservoir volume by measuring capacitance associated with component features of a delivery device. One aspect of the invention provides capacitance that varies dependent on the position of a bellows reservoir. Another aspect provides capacitance that varies dependent on the amount of propellant liquid within the delivery device housing. The amount of propellant liquid in the device housing varies with the reservoir volume because propellant liquid evaporates as the reservoir volume decreases. The invention provides volume-sensing capabilities without increase in volume or size of a delivery device. It also provides compatibility with Magnetic Resonance Imaging techniques and offers low power consumption. Still further, the invention provides reservoir volume sensing that compensates for asymmetrical positioning of a bellows reservoir.
According to a preferred embodiment, a capacitance is provided between an outer surface of a bellows reservoir, which acts as a first capacitor plate, and a conductive surface disposed proximate the bellows, which acts as a second capacitor plate. Preferably, the surface is provided on a conductive ring which surrounds the bellows. The ring is supported within the pump housing on insulated spacers. As the bellows moves from its extended full position to its collapsed empty position, the area of overlap, and therefore the capacitance between the first and second plates varies from a maximum value to a minimum value. An absorbent material is provided as a wick to absorb any liquid propellant in the pump housing to thereby prevent liquid propellant from affecting the capacitance between the bellows and the conductive
According to another aspect and preferred embodiment of the invention, a variable capacitor is provided with an absorbent material as a dielectric. The absorbent material is provided to absorb the liquid phase of the propellant in the pump housing and acts as a dielectric, having variable dielectric properties, between two stationary conductive plates provided in the housing. The amount of liquid propellant absorbed in the absorbent material varies with the reservoir volume. When the reservoir is in its full, expanded position, more liquid propellant is absorbed in the absorbent material. When the reservoir is in its compressed empty position, more of the propellant exists as vapor within the pump housing and therefore less liquid propellant is absorbed in the absorbent material. The dielectric properties of the capacitor are therefore higher and the capacitance is therefore maximized when the reservoir is in its full, extended position. Conversely, the dielectric properties are lower and the capacitance minimized when the reservoir is in its compact, empty position.
The objects, advantages novel features, and the further scope of applicability of the present invention will be set forth in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.